1. Field of the Invention
The present invention relates to an image data correction conversion circuit for correcting skew caused when a photoconductor is exposed to laser beams in an electrophotographic printer and a skew quantity correction method.
2. Description of the Related Art
Heretofore, in a printer using electrophotography, a photoconductor is exposed based upon image data and a latent image is formed. For exposure, a laser unit that irradiates a laser beam is often used. The above laser unit is acquired by integrating a laser and an optical system composed of a lens for focusing a laser beam from the laser and a polygon mirror.
The above laser unit is required to be installed without tilting for a photoconductor. For example, if the laser unit is installed right-handed downward in a horizontal scanning direction, the result of output shown in FIG. 5 is acquired. In FIG. 5, an arrow shows a direction in which image data is output and assuming that a right-hand upward direction is a positive direction and a right-hand downward direction is a negative direction, image data is output on printing paper with the image data skewed by xe2x80x98xe2x88x923xe2x80x99 in FIG. 5. The above quantity of skew of xe2x80x98xe2x88x923xe2x80x99 means that image data is skewed in a right-hand downward direction in a vertical scanning direction by 3 pixels in one scanning.
Referring to FIG. 5, an example of conventional type operation will be described below.
FIG. 5 shows the result of output in case an image data correction conversion circuit according to the present invention is not used. FIG. 5 shows a case that a laser unit is installed right-handed downward in a horizontal scanning direction of a photoconductor and the arrow shows a direction in which image data is output. Assuming that a right-hand upward direction is a positive direction and a right-hand downward direction is a negative direction, FIG. 5 shows that image data is output on printing paper with the image data skewed by xe2x80x98xe2x88x923xe2x80x99. If printing paper is divided into four blocks in a horizontal scanning direction and the blocks in which a first line (a line No. (1)) of image data is output are respectively D1-1, D1-2, D1-3 and D1-4, image data is sequentially printed right-handed downward with the image data skewed by xe2x80x98xe2x88x923xe2x80x99 as shown in FIG. 5. Afterward similarly, a line (2), a line (3), a line (4) and a line (5) are printed right-handed downward on printing paper.
However, in the above example of the conventional type, the larger the quantity of skew is, the worse the quality of printing of an output result becomes. Therefore, it can be said that the degree of skew has an effect upon the performance of a printer. Also, when a laser unit is installed, it is required to be precisely assembled to reduce the quantity of skew. Therefore, a problem that the manhour of the assembly is remarkably increased, a fixture and others are required to be produced and the price of a printer rises occurs.
The object of the present invention is to provide an image data correction conversion circuit for enabling the correction of skew as if the correction of skew was executed according to a program and a skew quantity correction method.
To achieve the above object, an image data correction conversion circuit disclosed in claim 1 is composed of a line buffer which can store image data for predetermined lines, CPU for measuring the quantity of skew of the image data stored in the line buffer and operating the number of blocks which divide the image data based upon the quantity of the skew, a control circuit for outputting a control signal to correct and convert the image data based upon data operated by CPU and a selector for selecting and outputting image data input to the line buffer and corrected by the quantity of the skew according to the control signal.
The above selector, control circuit and line buffer may be also integrated in a gate array.
The quantity of skew is measured when an image formation device in which the above image data correction conversion circuit is mounted such as a copying machine is powered on or if necessary. The control circuit may be also provided with a register for comparison for storing a counted dot clock value which functions as a measure to recognize a position every block operated by CPU in a horizontal scanning direction.
In this case, image data is sequentially stored in the above line buffer in synchronization with a dot clock generated corresponding to the image data. Image data input to the line buffer is defined corresponding to a counted dot clock value in an arbitrary position in a horizontal scanning direction. CPU operates the number of blocks which divide image data based upon the quantity of skew corresponding to a counted dot clock value and the operated counted dot clock value is stored in the register for comparison. The selector selects image data based upon the result of comparison between the counted dot clock value stored in the register for comparison and an actual counted dot clock value of image data.
Further, a controller for once storing image data before the image data is input to the line buffer and outputting the image data in synchronization with a dot clock may be also provided and a correction circuit may be also added to the gate array.
Also, it is desirable that the control circuit is provided with a line buffer control circuit for controlling the output of image data from the line buffer.
It is desirable that the above line buffer control circuit is provided with a comparator circuit for comparing a counted dot clock value stored in the register for comparison beforehand and a dot clock and a multiplexer control circuit for determining which block on which line of image data every line read from the line buffer is selected based upon the result of the comparison output from the comparator circuit and outputting the above image data switching the image data every block.
A skew quantity correction method according to the present invention is characterized in that image data for predetermined lines is stored, the quantity of skew of the stored image data is measured, the number of blocks which divide the image data is operated based upon the measured quantity of skew, the above quantity of skew is corrected based upon the operated data, the stored image data is selected and output.